It has been long known in the photographic industry of the value in obtaining stereo aerial photographs. The stereo photographs provide a three dimensional image in which vertical height information is present. This is obtained by viewing two overlapping photographs. It has also been recognized that a stereo radar image can also be obtained by viewing two overlapping sets of radar images, however, this has been difficult to achieve. In stereo photography the pair of photographs have the same illumination angles and therefor the same shadows because the angle of the sun does not change appreciatively between photographs. In radar, the illumination of the object is achieved by a transmitted pulse of electromagnetic energy. Currently stereo radar systems require two separate passes past the image area to gather both sets of radar images. In the first pass a pulse is transmitted to the surface and received by a receiver. On a subsequent pass a second pulse is transmitted at a different elevation angle than the first and is received by the receiver at a different location from the first recording. By transmitting two different pulses, two different sets of shadows and/or backscatter will result, unlike photography, in which the ifferences in the shadows will prevent the images from converging and resulting in poor image quality. If steep radar illumination depression angles are used to avoid these shadows, the accuracy of the imagery is sacrificed due to range compression. In order to avoid this problem, various techniques have been used.
The techniques which have been used and/or studied to collect stereo radar imagery involve multiple passes past the imaged surface with different ground range offsets. These offsets may be due to the rotation of the planet from pass to pass. This results in the undesirable effects of the amount of overlapping coverage and the angular viewing difference being a strong variable depending on the eccentricity of the orbit, the location of the satellite in this orbit, and the circularity of the orbit. Thus the vertical scale will be variable and it will have to be recalculated for each new orbital condition. Another technique involves redirection of the antenna on a later orbit to change the viewing angle. Still another technique requires that the first pass transmits and receives the radar signal while a second pass collects the vertical height information with a radar altimeter. All of these techniques require processing data from more than one orbit to provide complete data for determining accurately the ground position of surface features. Most of the techniques are limited to certain orbits and can not give complete data for portions of these orbits.
The maximum angular difference between receivers is limited to the vertical angular beamwidth of the antenna. Larger angular differences, due to the planetary rotation between orbital passes eliminates all overlapping coverage, and therefore, the stereo imagery, while smaller displacement gives more overlapping coverage, but with progressively less angular difference. Thus, currently the stereo imagery obtained is limited to only those very small portions of the orbit which provide orbital pass to orbital pass displacements in the limited ranges of the angular coverage of the antenna. For any given planetary orbit orientation there are large portions of the planet's surface which cannot be stereo imaged. Re-aiming the antenna between orbital passes can increase the angular difference and give total overlapping coverage for portions of the orbit, but this requires complex and precise antenna pointing control while providing stereo imagery only for a limited range of latitudes. Complete coverage of even these latitudes requires many orbital passes.
Several techniques have been suggested for providing single flight stereo radar techniques involving aircraft, but not for spacecrafts. Furthermore, these techniques have many of the same disadvantages as discussed above. In these, a single aircraft is flown while transmitting two different radar beam pattern characteristics. This technique requires the use of two fan beam patterns to generate parallax on the images (see G. E. Carlson, "PERFORMANCE COMPARISON OF TECHNIQUES FOR OBTAINING STEREO RADAR IMAGES," IEEE TRANSACTIONS ON GEOSCIENCE ELECTRONICS, VGE-12, 114-122, (1974)).
A fairly thorough discussion of the above various methods is set forth in F. W. LEBERL, SATELLITE RADARGRAMMETRY--PHASE I, TECHNICAL UNIVERSITY AND GRAZ RESEARCH CENTER, 1982.
This invention provides for a single pass radar system utilizing steep viewing angles while eliminating most of the variables with fixed values independent of orbit. The vertical scale would be fixed and a function only of the range coverage and the altitude of the orbit. This system can produce full imaged swath width stereo imagery for any surface position from any given imaging orbit because the angular difference required is self-generated and is not dependent upon the planet's rotational motion. Such a radar system would be very useful for future earth resources surveys, continuous monitoring of earth resources and military all weather targeting missions.